Users of hearing protection devices (HPDs), including the communication headsets and helmets commonly worn by aircraft air and ground crews, military personnel, and emergency responders, and persons in industry, mining and construction working in noisy environments, have long complained that HPDs interfere with their ability to communicate and to hear warning signals. The technology of active noise control together with signal processing applied to a communication channel built into the HPD is proposed as a potential solution. The combination could improve noise reduction at low frequencies and enhance the intelligibility of speech reproduced by the communication channel, while maintaining the overall noise reduction within occupational exposure requirements and the perception of an warning sound outside the HPD. Research to achieve these goals is proposed based on an active circumaural earmuff equipped with miniature earphones and microphones, and independent frequency bands of signal processing. Various levels of signal processing complexity are considered to optimize the speech signal-to-noise ratio, and reduce the influence of the upward spread of masking. The demonstration of "proof of principle" is preceeded by the establishment of subject based (subjective) and instrument based (objective) metrics for determining active and passive noise reduction, speech intelligibility, and the perception of warning sounds (subjective method only). The objective metrics, which provide approximate results in a fraction of the time required for subjective measurements, are to serve as surrogates for the corresponding subjective metrics during device development. Users of hearing protection devices (HPDs), including the communication headsets and helmets commonly worn by aircraft air and ground crews, military personnel, and emergency responders, and persons in industry, mining and construction working in noisy environments, have long complained that HPDs interfere with their ability to communicate and to hear warning signals. Research is proposed to demonstrate "proof of principle" of a specialized HPD designed specifically to: 1) improve speech intelligibility of a built-in communication channel; 2) maintain the perception of alarm signals external to the HPD, and; 3) maintain attenuation of environmental noise. [unreadable] [unreadable] [unreadable]